Planning to retire on time? Have you picked a number (age or dollars) as a target? You might pick the “full” Social Security eligibility age of 67, but even that’s not set in stone. Some people take the benefit early; others have the luxury of waiting a few years and getting a higher payment. Even so, safe retirement investing is not an easy calculation to make.

Many people have retirement thrust upon them. Downsizing can strike at any point along the line in your 50s or early 60s. Companies cast off pricey managers and experienced technical workers and only sometimes rehire, and often then as short-term contractors.

If you’re nearing that age and think you’ll start your safe retirement investing plan next year, or the year after…watch out.

It’s nice to think that you will have a steady glide path to a predictable, leisurely decision. The truth is, any change as big as retirement is fraught with anxiety, even if you have the money to make the move. It’s only natural.

Once upon a time, longer life spans paid a clear demographic dividend: More kids made it to adulthood where they could produce goods and services and more younger adults survived. That meant more working age folks, and that led to higher economic output per capita.

But something different is happening now. “Instead of additional years of life being realized early in the life cycle, they are being realized late in life,” Stanford University economists Karen Eggleston and Victor Fuchs write in the current issue of the Journal of Economic Perspectives with the usual complement of charts and tables.

The article continues, explaining that a century ago about 20% of the increase in life expectancy was experienced by folks over 65. By the end of the 20th century, however, people past that age mark were seeing more than 75% of the gains in longevity. We started living longer.

Put another way, the Centers for Disease Control figures that a newborn today is likely to live 78.5 years. However, if you make it to 65, you are on average likely to live another 19.2 years. That’s close to 85 years old. And it’s an average, so some of us clearly will exceed the implied top end. Getting to 100 suddenly seems real, doesn’t it?

Safe retirement investing the easy way

Perhaps a better way to think about safe retirement investing is not in years or dollars but in terms of the percentage of your life that you expect to produce income and the percentage not producing it. Unless you manage to make a lot of money fast and early, it’s hard to imagine how a person can expect to work for 25 years and then finance an additional 35 years of safe retirement investing income. The ratio is lopsided: earning five to pay for seven.

If you aim to retire at 65, that would be 40 years of labor (assuming you start working and saving soon after the traditional college graduation age) and then perhaps 20 years of retirement. The ratio is suddenly reasonable: earning four to pay for two.

Likewise, sustaining that compound-interest effect as you draw down in retirement could be the difference between making it comfortably to the far end of the actuarial table — or becoming unexpectedly dependent upon others as you age.

MarketRiders is web-based, do-it-yourself investing software that lets you build and manage a globally diversified low-cost retirement portfolio using methods developed by Nobel Laureates. Reduce your investment fees, get Wall Street out of your pocket, and retire with more.